28188721 has 4 divisors (see below), whose sum is σ = 30751344. Its totient is φ = 25626100.

The previous prime is 28188697. The next prime is 28188757. The reversal of 28188721 is 12788182.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 28188721 - 2¹¹ = 28186673 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (28188791) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1281295 + ... + 1281316.

It is an arithmetic number, because the mean of its divisors is an integer number (7687836).

Almost surely, 2^28188721 is an apocalyptic number.

It is an amenable number.

28188721 is a deficient number, since it is larger than the sum of its proper divisors (2562623).

28188721 is a wasteful number, since it uses less digits than its factorization.

28188721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2562622.

The product of its digits is 14336, while the sum is 37.

The square root of 28188721 is about 5309.3051334426. The cubic root of 28188721 is about 304.3395941006.

The spelling of 28188721 in words is "twenty-eight million, one hundred eighty-eight thousand, seven hundred twenty-one".